River Beds And Glacial Melt Carve Similar Landscapes

Summer erosion driven by flowing meltwater from atop Greenland's ice sheet toward the sea creates landscapes similar to drainage basins carved on land by the Willamette and McKenzie rivers, says University of Oregon geologist Leif Karlstrom. Image courtesy: University of Oregon.

The research should help broaden scientific understanding of melt rates and improve projections about glacial response to climate change. The finding, published in a paper in Geophysical Research Letters, came as a surprise to scientists.

By Jim Barlow, University of Oregon

Summer erosion driven by flowing meltwater from atop Greenland’s ice sheet toward the sea creates landscapes similar to drainage basins carved on land by the Willamette and McKenzie rivers, says University of Oregon (UO) geologist Leif Karlstrom.

The finding, published in a paper in Geophysical Research Letters, came as a surprise, he said. That’s because on ice the formation of drainage basins is done with meltwater alone, while on land rivers erode the landscape by pushing and plucking sediment as they flow, cutting down as the land surface uplifts due to tectonic activity.

The discovery also carries deeper meaning.

The approach used to study the ice sheet should help broaden scientific understanding of melt rates and improve projections about glacial response to climate change, said Karlstrom, an assistant professor in the Department of Geological Sciences.

“How fast is the ice sheet melting, and how much the melt will contribute to rising sea levels are important questions,” he said. “It is important to quantify the melt rate, but that is not easy. Our study allows us to use geometric characteristics of the channel network — their patterns on the landscape — as a diagnostic tool.”

Projections on sea-level rise, such as those done with remote sensing or satellite observations, he said, are problematic because melt rates vary each year, based on such factors as summer temperatures and elevations across the ice sheet.

In the study, Karlstrom and Kang Yang of the University of California, Los Angeles, analyzed high-resolution satellite imagery from NASA digital elevation models. The images let them see slopes of the ice sheet and underlying bedrock. They focused on stream channels at four levels of the ice sheet, from 1,000 meters (3,280 feet) to 1,600 meters (5,249 feet), of southwest Greenland.

“It’s lower elevations at the margins of the ice sheet that experience the most melt,” Karlstrom said. River erosion stops each year when freezing temperatures return. Frozen channels from previous years remain visible, providing a yearly history of erosion patterns much like tree rings reflect age, he said.

Geologists who study geomorphology — how landscapes form — such as the UO’s Joshua Roering, who was consulted on the study, now have a virtual real-time model to test theories of landscape evolution, Karlstrom said. River erosion on land occurs over millions of years, but streams on the ice sheet carve their routes much more rapidly. (See “Related Links” for more on Roering’s work.)

Abstract

Supraglacial stream networks incise via thermal erosion of underlying ice, reflecting a balance between localized fluvial incision and dynamic topography from underlying ice flow. We analyze high-resolution digital elevation models of the ice surface and bedrock in the southwest Greenland Ice Sheet from 1000-1600 m elevation to quantify the importance of fluvial erosion. At wavelengths greater than ice thickness, bedrock dominates surface topography so supraglacial drainage basins are fixed spatially. At smaller wavelengths, fluvial erosion significantly affects topography. Stream longitudinal profiles exhibit positive mean curvature and consistent power law scaling between local channel slope and drainage area, suggestive of adjustment toward topographic steady state. We interpret these observations with a model for fluvial thermal erosion on top of a flowing ice substrate that predicts concave up steady state longitudinal profiles, where average concavity is most sensitive to melt rate and the relative magnitudes of ice flow and fluvial erosion.